"It wasn't the same person that says I love you. It was not those... eyes," she told Sawyer in an exclusive interview. "He had ... no soul in his eyes. Just blank. ...He was clearly blacked out. There was no person when I looked at him."

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The February 2009 assault left the 21-year-old battered and bruised, and Brown was sentenced to five years of probation, community labor, and one year of domestic-violence counseling.

"All I kept thinking all the time: When is it going to stop? When it is going to stop?" she said.

After the attack, Rihanna briefly reunited with Brown, but said that she never forgave him.

"I just said to him, 'I can't do this.' I resented him. I resented him so much. And I always put the tough face on and try to, I can do anything face, and just try to play it off. But he knew. He knew it. He kept asking me, 'You hate me, don't you? You hate me,'" she told Sawyer. "And I would lie and I would say, 'No, no. And ... I did hate him. ... Everything about him annoyed me. So finally ... I just said, we can't ... we can't do this. I cannot continue to do this."

Budding Romance Spirals Out of Control

Before the dramatic incident, the couple's seemingly fairytale relationship began as a friendship. A 16-year-old Rihanna -- with a growing empire of endorsements, fashion spreads, and a bank account in the multi-millions -- said she was attracted to 15-year-old Brown's personality, humor and charm.

"We were just friends. We always played. So it was ... a good feeling to come out of the adult lifestyle and just, when you're in your room, just be yourself," she said of their year-and-a-half-long romance. "He was definitely my first big love."

But the starlet said that the deeper they fell for one another, the darker their relationship became, bordering on an "obsession."

"To fall in love with your best friend it ... can be scary because the ... the emotions get really, they get the best of you. Like it takes over," she said. "The more in love we became, the more dangerous we became for each other, equally as dangerous."

CLICK HERE for resources and information on how to help stop domestic violence among teens and young women.

Rihanna Recounts Details of Assault

Rihanna recounted the details of the "ugly" night, describing how she and Brown got in a heated argument over a text message on Brown's phone.

"I caught him in a lie. And he wouldn't tell the truth ... I was being more annoyed at that point in our relationship he had to lie about something so stupid," she said. "I couldn't take that he kept lying to me. And he couldn't take that I wouldn't drop it. And ... it was ugly."

According to the police affidavit, Brown shoved Rihanna into the window of his car, while driving. He punched her several times in the eye and said, "I'm going to beat the s*** out of you when we get home." When Rihanna tried to call her assistant's phone, Brown warned, "You just did the stupidest thing ever. I'm going to kill you," and threw her phone out the window.

Rihanna confirmed the brutal account, but told Sawyer she knew Brown wouldn't act on his words. "I know he was saying it to scare me," she said.

Rihanna acknowledged that Brown bit her and had her in a headlock until she had trouble breathing. She told Sawyer that she did not try to fight back.

"I fended him off with my feet from one ... side of the car, but ... it was not like, it was not like a fight with each other. I just ... I really just wanted it to stop," she said.

Her screams prompted someone to call 911 and report the disturbance to the Los Angeles Police Department. Rihanna told Sawyer that, by that point, she was "battered."

"I was bleeding. I was swollen in my face," she said. "So there was no way of me getting home, except for, my next option was to get out of the car and walk. Start walking in a gown, in a bloody face. So I really don't know what my plan was. I didn't have a plan. That whole night was not part of my plan."

Rihanna: 'I Made a Selfish Decision for Love'

To the shock of many fans, the two pop stars reunited after the assault -- a decision which Rihanna now calls a mistake.

"I went through ... a host of emotions. It was confusing for me. I was still attached by love. But I wasn't thinking about ... the reality of the situation," she said.

In denial, she said her love for Brown clouded her judgment. "I made a selfish decision for love. It was a wakeup call. ...Love is so blind. It is so blind.

"You start lying to yourself. ... This is a memory you don't want to have ever again. ... the physical wounds go away, you put it in the back of your head and you start lying to yourself subconsciously," she said. "I felt very lonely. ...I couldn't even go back to my own house because there were 200 people outside with cameras, paparazzi, journalists, fans, neighbors."

She told Sawyer that the media blitz was overwhelming and extremely isolating.

"This is why I made my decision. ...I felt really lonely. ...There were times when I cried. There were times when I just sat there all day and watched TV," she said.

Rihanna said she was "embarrassed" and "humiliated" by the graphic photo of her that was leaked to the press after Brown's attack, in which her face appears battered and swollen.

"Who likes seeing their face like that? ...I feel humiliated. I get angry, all over again, every time I see it," she said. "The whole thing plays back in my head. So I don't like to see it."

Rihanna told Sawyer that at the time, she even convinced herself that she had to protect Brown from scrutiny.

"If I feel this depressed, then what is he going through?" she said. "I had to protect him. I thought that I had to let him know, don't do anything crazy. Like just hang in there...The whole world hates him now. His fans, his career. He just, he lost me, I just need to let him know; don't do anything stupid ...I'm not saying that's an excuse for me to go back, but this is what I was thinking about."

The singer said she replayed the incident in her head, and began to question what she could have done to set off Brown.

"Initially you start thinking, what could I have possibly have said to make him hit me?" she said. "Eventually you're like, you know ... stop. Stop. Stop. Ask yourself. There's nothing you can do or say to make somebody do that to you. That's on them. And ... I just knew he had a problem. I knew he had a problem. He had a temper. He needed to get some help. And ... and he did."

According to psychologists, returning to an abuser is not unusual for victims of domestic violence. "I'm a human being and people put me on a very unrealistic pedestal. And all these expectations, I'm not perfect," she said.

"The thing that men don't realize, when they hit a woman, it's... the face, the broken arm, the black eye, it's going to heal. That's not ... the problem. It's the scar inside," she said. "You flashback. You ... you remember it all the time. It comes back to you whether you like it or not. And it's painful. So I don't think he understood that. They never do."

Shortly after the March trip to Miami with Brown, she realized she made a mistake and broke things off.

"The past few months, I just, I didn't talk about it to anyone, to no one. Not my friends. Not my family … It's not something that I wanted to relive. It's not something I wanted to think about ... I just put it away in a box...and just ignored it really."

Rihanna Returns: Her Message for Young Women

The star, who has sold 15 million albums, -- is back after a nine-month hiatus with a new single, "Russian Roulette," from her highly-anticipated album "Rated R" to be released later this month.

"I feel when I look back on the four years, I feel like I've grown so much ... in such a short space of time and achieved a lot," she said. "I'm blessed."

The singer told Sawyer that she is not dating anyone now and can't imagine getting back together with Brown.

"I don't have a desire at all to be with him," she said. "... I can't see how we ... would get back together, but I'm also not God and I can't predict the future."

Rihanna decided to speak publicly about the incident to warn others who may be in danger of returning to abuse.

"I knew I had to do this in order to move on for me and in order for my fans to move on with me because it would always be a question in their mind," she said. "I don't want that five years from now every time they see Rihanna, they think of Chris Brown beating me. That's not who I am. It's just one thing that happened to me."

CLICK HERE for resources and information on how to help stop domestic violence among teens and young women.

Watch the full interview with Rihanna on "20/20" at 10 p.m. E.T. Friday, Nov. 6.